There have been known hydro-mechanical (power split type) transmissions (HMTs) which hydraulically transmit part of input power while mechanically transmitting the remaining part of it. Since mechanical power is transmitted with high efficiency, hydro-mechanical transmissions (HMTs) are designed to convert only part of mechanical power into hydraulic power, so that they can achieve high transmission efficiency. By virtue of this, they are considered to be ideal transmissions for vehicles subjected to wide load variations such as bulldozers and wheel loaders and some of them are, in fact, employed in such vehicles.
In a typical hydro-mechanical transmission (HMT), variable speed characteristics are achieved by a planetary gear train. More concretely, the following arrangement is employed. Of three elements of the planetary gear train (i.e., the sun gear, the carrier provided with planetary gears, and the ring gear), a first element is coupled to the input shaft, a second element is coupled to the output shaft, and a third element is coupled to a hydraulic pump or hydraulic motor. The rotational speed of the hydraulic pump or hydraulic motor is varied thereby changing the rotational speed of the output shaft.
In the known art, there are basically two types of HMTs. One is an “output-split type” where a hydraulic pump or hydraulic motor, which is connected by means of a hydraulic circuit to another hydraulic pump or hydraulic motor coupled to the planetary gear train, is coupled to the input shaft of the transmission so as to obtain a constant speed ratio. The other is an “input-split type” where a hydraulic pump or hydraulic motor, which is connected by means of a hydraulic circuit to another hydraulic pump or hydraulic motor coupled to the planetary gear train, is coupled to the output shaft of the transmission so as to obtain a constant speed ratio. Further, the output-split type and input-split type are respectively classified into six types according to which of the three elements of the planetary gear train is coupled to the hydraulic pump, hydraulic motor or input/output shafts and, in total, 12 types are available as basic combinations.
The conventional output-split type HMT and input-split type HMT will be respectively described in more detail.
FIG. 5(a) shows a schematic structural diagram of an output-split type HMT. In this output-split type HMT 100, a first gear 103 is secured to an input shaft 102 to which power from an engine 101 is input. A second gear 104 meshing with the first gear 103 is secured to a shaft 105a of a first pump/motor 105. Secured to the input shaft 102 is a sun gear 107 of a planetary gear train 106. A plurality of planetary gears 108 are disposed so as to mesh with the periphery of the sun gear 107. Each planetary gear 108 is axially supported by a planetary carrier 109 to which an output shaft 110 is secured. A ring gear 111 meshes with the periphery of the planetary gear set 108. Meshing with the periphery of the ring gear 111 is a third gear 112 which is secured to a shaft 113a of a second pump/motor 113. In this arrangement, the first pump/motor 105 is hydraulically connected to the second pump/motor 113 by a piping 114.
In such a system, when the rotational speed of the second pump/motor 113, that is, the rotational speed of the ring gear 111 is zero, hydraulically transmitted power becomes zero so that all power is transmitted through the mechanical unit. On the basis of the rotational speed of the output shaft 110 at that time, the operation of this system will be described.
(1) When increasing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power through the medium of hydraulic pressure and is activated to increase the rotational speed of the output shaft 110. At that time, the first pump/motor 105 serves as a pump whereas the second pump/motor 113 serves as a motor, so that energy is transmitted from the first pump/motor 105 to the second pump/motor 113 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes plus (+) as indicated by line A-B in FIG. 5(b) and the hydraulic power flows in a forward direction, i.e., from the input shaft 102 toward the planetary gear train 106.
(2) When reducing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power from the planetary gear train 106 and rotates in a direction opposite to that of the case (1). At that time, the second pump/motor 113 serves as a pump whereas the first pump/motor 105 serves as a motor, so that energy is transmitted from the second pump/motor 113 to the first pump/motor 105 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes minus (−) as indicated by line A-C in FIG. 5(b) and the hydraulic power flows in a reverse direction, i.e., from the planetary gear train 106 toward the input shaft 102.
FIG. 6(a) shows an input-split type HMT 200 in which the planetary gear train 106 is disposed on the side of the input shaft 102 whereas the first pump/motor 105 is disposed on the side of the output shaft 110. In FIG. 6(a), the parts that are substantially equivalent or function substantially similarly to those of the transmission 100 shown in FIG. 5(a) are indicated by the same numerals as in FIG. 5(a), and a detailed explanation of them is skipped herein.
The input-split type transmission 200 is constructed as follows.
(1) When increasing the rotational speed of the output shaft 110, the second pump/motor 113 serves as a motor while the first pump/motor 105 serves as a pump, so that energy is transmitted from the first pump/motor 105 to the second pump/motor 113 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes minus (−) as indicated by line A-D in FIG. 6(b) and the hydraulic power flows in a reverse direction, i.e., from the output shaft 110 toward the planetary gear train 106.
(2) When reducing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power from the planetary gear train 106 and rotates in a direction opposite to that of the case (1). At that time, the second pump/motor 113 serves as a pump whereas the first pump/motor 105 serves as a motor, so that energy is transmitted from the second pump/motor 113 to the first pump/motor 105 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes plus (+) as indicated by line A-E in FIG. 6(b) and the hydraulic power flows in a forward direction, i.e., from the planetary gear train 106 toward the output shaft 110.
As such, in both output-split type and input-split type transmissions, energy flows in forward and reverse directions occur in the speed increasing region and the speed reducing region. The transmission efficiency of energy in this case will be hereinafter examined, taking the output-split type HMT 100 shown in FIG. 5 for example. Herein, the transmission efficiency of the mechanical unit is 95% and the transmission efficiency of the hydrostatic unit is 80% (Generally, where pump-motors are used, transmission efficiency is low). For easy comparison, assume that the amount of engine power is 1.0 and one third the engine power is input to the hydrostatic unit.
FIG. 7(a) shows the case where hydraulic power flows in the forward direction. Specifically, one third (0.333 part) the energy output from the engine 101 flows to the hydrostatic unit for increasing speed. Transmitted to the output shaft 110 are 0.633 (=(1−⅓)×0.95) part of energy from the mechanical unit and 0.267 (=0.333×0.8) part of energy from the hydrostatic unit. As a result, the overall efficiency becomes 0.9 (=0.633+0.267). The case where hydraulic power flows in the reverse direction is shown in FIG. 7(b). In this case, 1.267 (=1+0.267) parts of energy are input to the mechanical unit and 1.20 (=1.267×0.95) parts of energy are transmitted, so that the overall efficiency is 0.870 (=1.20−0.333).
As just described, when hydraulic power flows in the reverse direction, a large flow of energy occurs in each element, resulting in poor efficiency. In other words, a forward flow of hydraulic energy is better than a reverse flow of hydraulic energy. As seen from FIGS. 7(a) and 7(b), if part of energy flows in the reverse direction, the energy that pass through the mechanical unit will increase, and therefore, there arises a need to increase the size of the planetary gear train, which leads to a disadvantage in economical efficiency.
As an attempt to solve the problems of the prior art output-split type HMT and input-split type HMT, there has been proposed a transmission capable of serving as an output-split type HMT when the rotational speed of the output shaft is increased and as an input-split type HMT when the rotational speed of the output shaft is reduced. This proposed transmission has several advantages: the horsepower transmitted by hydraulic power can be kept to be plus irrespective of the rotational speed of the output shaft, so that hydraulic power can be allowed to constantly flow in the forward direction and increased energy efficiency can be achieved in all speed zones including the low speed zone to high speed zone.
In a vehicle having the above transmission capable of selectively functioning as an output-split HMT or as an input-split HMT, when the transmission is shifted from forward to reverse or vise versa, the following operation is usually carried out: After disengagement of the forward (or reverse) clutch, the reverse (or forward) clutch is allowed to slide, so that the movement of the vehicle is changed from forward (or reverse) to reverse (or forward) and the reverse (or forward) clutch is engaged.
However, such sliding engagement of the forward or reverse clutch during operation for shifting the transmission of the vehicle between forward and reverse causes higher load and heat load on the forward and reverse clutches, with higher vehicle speed before initiating the gear shifting operation. As a result, it becomes necessary to increase the capacity of the clutches.
The invention is directed to overcoming the foregoing shortcomings and a primary object of the invention is to provide a control system for a hydro-mechanical transmission which selectively functions as an output-split type HMT or as an input-split type HMT, the control system being capable of reducing the load imposed on the forward and reverse clutches during gear shifting between forward and reverse.